Could not load type 'System.Runtime.CompilerServices.ExtensionAttribute' from assembly 'mscorlib

Could not load type 'System.Runtime.CompilerServices.ExtensionAttribute' from assembly mscorlib

Yes, this technically can go wrong when you execute code on .NET 4.0 instead of .NET 4.5. The attribute was moved from System.Core.dll to mscorlib.dll in .NET 4.5. While that sounds like a rather nasty breaking change in a framework version that is supposed to be 100% compatible, a [TypeForwardedTo] attribute is supposed to make this difference unobservable.

As Murphy would have it, every well intended change like this has at least one failure mode that nobody thought of. This appears to go wrong when ILMerge was used to merge several assemblies into one and that tool was used incorrectly. A good feedback article that describes this breakage is here. It links to a blog post that describes the mistake. It is rather a long article, but if I interpret it correctly then the wrong ILMerge command line option causes this problem:

  /targetplatform:"v4,c:\windows\Microsoft.NET\Framework\v4.0.30319"

Which is incorrect. When you install 4.5 on the machine that builds the program then the assemblies in that directory are updated from 4.0 to 4.5 and are no longer suitable to target 4.0. Those assemblies really shouldn't be there anymore but were kept for compat reasons. The proper reference assemblies are the 4.0 reference assemblies, stored elsewhere:

  /targetplatform:"v4,C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETFramework\v4.0"

So possible workarounds are to fall back to 4.0 on the build machine, install .NET 4.5 on the target machine and the real fix, to rebuild the project from the provided source code, fixing the ILMerge command.

Do note that this failure mode isn't exclusive to ILMerge, it is just a very common case. Any other scenario where these 4.5 assemblies are used as reference assemblies in a project that targets 4.0 is liable to fail the same way. Judging from other questions, another common failure mode is in build servers that were setup without using a valid VS license. And overlooking that the multi-targeting packs are a free download.

Using the reference assemblies in the c:\program files (x86) subdirectory is a rock hard requirement. Starting at .NET 4.0, already important to avoid accidentally taking a dependency on a class or method that was added in the 4.01, 4.02 and 4.03 releases. But absolutely essential now that 4.5 is released.


I had this problem, except the type it couldn't load was System.Reflection.AssemblyMetadataAttribute. The web application was built on a machine with .NET 4.5 installed (runs fine there), with 4.0 as the target framework, but the error presented itself when it was run on a web server with only 4.0 installed. I then tried it on a web server with 4.5 installed and there was no error. So, as others have said, this is all due to the screwy way Microsoft released 4.5, which basically is an upgrade to (and overwrite of) version 4.0. The System.Reflection assembly references a type that doesn't exist in 4.0 (AssemblyMetadataAttribute) so it will fail if you don't have the new System.Reflection.dll.

You can either install .NET 4.5 on the target web server, or build the application on a machine that does not have 4.5 installed. Far from an ideal resolution.


I had this exact same problem with a site (Kentico CMS), starting development in 4.5, finding out the production server only supports 4.0, tried going back down to target framework of 4.0. Compiling the other posts in this thread (specifically changing target framework to .Net 4 and .Net 4.5 still being referenced). I searched through my solution and found that a handful of the NuGet packages were still using libraries with targetFramework="net45".

packages.config (before):
<?xml version="1.0" encoding="utf-8"?>
<packages>
  <package id="AutoMapper" version="3.1.0" targetFramework="net45" />
  <package id="EntityFramework" version="5.0.0" targetFramework="net45" />
  <package id="Microsoft.AspNet.WebApi.Client" version="5.0.0" targetFramework="net45" />
  <package id="Newtonsoft.Json" version="4.5.11" targetFramework="net45" />
</packages>

I changed the projects target framework back to 4.5, removed all NuGet libraries, went back down to 4.0 and re-added the libraries (had to use some previous versions that were not dependent on 4.5).

packages.config (after):
<?xml version="1.0" encoding="utf-8"?>
<packages>
  <package id="AutoMapper" version="3.1.1" targetFramework="net40" />
  <package id="EntityFramework" version="6.0.2" targetFramework="net40" />
  <package id="Microsoft.AspNet.WebApi.Client" version="4.0.30506.0" targetFramework="net40" />
  <package id="Microsoft.Net.Http" version="2.0.20710.0" targetFramework="net40" />
  <package id="Newtonsoft.Json" version="4.5.11" targetFramework="net40" />
</packages>

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